1. Field of the Invention
This invention relates in general to the field of pressurized light water nuclear reactors and in particular to the fuel assemblies of such nuclear reactors and particularly those intended for commercial use.
2. Description of the Prior Art
It is well known that commercial pressurized light water nuclear reactors are both a technical and commercial success. In very simple and generalized terms, pressurized water nuclear reactors include independent primary and secondary flow systems. The primary system is usually a closed system using light water as the reactor coolant and the nuclear moderating fluid. The light water which is pressurized to prevent boiling, is heated upon flowing through a nuclear core which is contained within a pressure vessel. The heated reactor coolant upon leaving the pressure is ducted through a steam generator where it transfers the heat acquired from the nuclear core to another fluid within the secondary system. The cooled reactor coolant then flows back into the pressure vessel to be reheated to continue the cycle. The fluid within the secondary system, most often, is also light water, which is converted into steam within the steam generator. The steam is then used to generate electricity by large commercial steam turbine-electrical generator combinations.
The nuclear core is housed by and structurally supported within the pressure vessel. The pressure vessel also includes apparatus which ducts the primary reactor coolant from inlet nozzles attached to the pressure vessel, through the nuclear core and then out of the pressure vessel through outlet nozzles.
The nuclear core in reactors of the prior art to which the present invention applies, comprises a plurality of elongated fuel assemblies stacked side-by-side against each to form an irregular but approximately round cylindrical configuration. Each fuel assembly includes an inlet nozzle, an outlet nozzle and a square array of elongated nuclear fuel rods separated from each other and structurally supported by a number of grids spaced along the axial length of the elongated fuel rods. The inlet and outlet nozzles are not connected to the nuclear fuel rods. Rather, they are connected to respective ends of elongated guide tubes which are interspersed among the fuel rods. The guide tubes comprise hollow tubes which are usually larger in diameter than the diameter of the fuel rods. The fuel rods are, of course, positioned between the inlet and outlet nozzles. The primary coolant enters the fuel assembly through the inlet nozzle, flows past the fuel rods, and out the outlet nozzle.
The guide tubes interspersed among the fuel rods provide a guidance means by which control rods may be inserted within some of the fuel assemblies. The control rods are used to control the fission process of the nuclear core so that a sustained nuclear reactor is maintained. Typically, the fuel rods are loaded with enriched uranium. Absorber rods which also fit within the guide tubes in other fuel assemblies are used to absorb the excess neutrons produced by the enriched uranium. As the amount of enrichment decreases, from being burned during reactor operation, the absorber rods are withdrawn to compensate for the decrease in enrichment. Enrichment, although very expensive, is used to provide a means whereby the life of the core is extended in order to maximize the time between refueling of the core. Refueling requires shutdown of the reactor which is not only time consuming but also comprises a period of downtime when no electricity is being produced. Hence, an extended core life is extremely advantageous and desirable.
The benefits of an extended core life in the prior art is not fully attained because of incomplete fuel consumption in the axial and radial directions during the life of the core. The incomplete consumption of fuel is most prevalant at the top and the bottom of the core. Attempts have been made to alleviate this poor utilization of fissile material inventory by employing natural uranium or, seed blankets, at the top and the bottom of the core. In theory, enriched uranium can be removed and replaced by the seed blankets of natural uranium. Thus, there would be a savings of enriched uranium in areas where there is incomplete burning and the natural uranium would produce plutonium which can thereafter be burned. The benefits of such attempts, however, have not been as successful as anticipated because the production of the plutonium in the natural uranium blankets cannot be utilized as well as desired at th.e ends of the core. Also, a significant and detrimental condition of axial power peaking increase occurred because the active core "looks" shorter with the relatively nonfissionable natural uranium at the core ends.
Another prior art attempt to alleviate the noncomplete fuel consumption at the ends of the core was to separate the core into two axial sections, a lower assembly and an upper assembly. After a specified period of reactor operation, for example, when the anticipated core life has been reduced by a predetermined percent, or the fuel is depleted by a predetermined amount, the lower and upper assemblies are interchanged causing the core ends to be turned toward the middle of the core. In this manner, it was anticipated that the nonburned core ends would then be relocated to a core region where full consumption would occur. While such attempts do enhance more full utilization of the fuel inventory, it also causes prohibitive axial power peaking. And, significant undesirable reactivity loss is experienced because of a fuel gap which occurs between the upper and lower core assemblies.
The disadvantages associated with the prior art attempts to alleviate the problem of incomplete fuel consumption at the core ends has resulted in the nonemployment of these designs in commercial nuclear power plants. Thus, there still exists a need for a design which: better utilizes the fuel at the core ends such that substantially full consumption of the fissile material in the core occurs at the end of core life; allows the use of axial blankets for plutonium production and burnup; allows the use of lower enriched fuel; and, provides axial power flattening.
Accordingly, a primary object of the present invention is to provide a high utilization fuel assembly which provides for full consumption of the fissile material over the entire length of the core.
Another primary object of the present invention is to provide a high utilization fuel assembly which provides for the use of axial uranium blankets and fissile material production at the core ends.
Another primary object of the present invention is to provide a high utilization fuel assembly which allows the use of lower enriched uranium without sacrificing extended core life.
Another primary object of the present invention is to provide a high utilization fuel assembly which provides for axial power flattening of an inverted, two section core.
Although not specifically listed above, there are other objects of the invention which will be apparent to those skilled in the art, which other objects are intended to be achieved by the present invention.